Excited-state Intramolecular Proton Transfer Research Articles

Uncovering Integrated Dual-State ESIPT-Conductivity, Redox-Capacity, and Opto-Electronic Responses Toward Hg(II)/ Cr(III) of Aliphatic Fluorescent Polymers.

Excited-state intramolecular proton transfer (ESIPT)-associated dual-state emissive aliphatic dual-light emitting conducting polymers (DLECPs) having oxidation-reduction capacities are prepared polymerizing 2-acrylamido-2-methylpropane-1-sulfonic acid, methacrylic acid, and 2-methyl-3-(N-(2-methyl-1-sulfopropan-2-yl)acrylamido)propanoic acid monomers. Of as-synthesized DLECPs, nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies, fluorescent enhancements (I/I0), and computational investigation indicate intriguing photophysical features in DLECP3 (optimum composition). In DLECP3, ─CONH─, ─CON<, and ─COOH subluminophores are recognized by density-functional theory (DFT)/time-dependent-DFT calculations and experimental investigations. ESIPT-associated dual-state emission/conductivity, aggregation-enhanced emissions, selective opto-electronic responses toward Hg(II)/Cr(III) at 437/574nm, and redox properties of DLECP3 are supported by solid-state/solution spectroscopies, time-correlated single photon counting (TCSPC) measurements, dual-state excitation dependent emissions, microscopic images, electrochemical measurements, and DFT calculations. Here, preferential interaction of Hg(II)/Cr(III) with DLECP3 (amide)/DLECP3 (imidol) and reduction/oxidation of Hg(II)/Cr(III) to Hg(I)/Cr(VI) are substantiated by UV-vis, FTIR, and X-ray photoelectron spectroscopies; TCSPC measurements; NMR-titration; electrochemical studies; alongside computational calculations. The proton-electrical conductivities of DLECP3, Hg(II/I)-DLECP3, and Cr(III/VI)-DLECP3 in solids/solutions are 15.27 × 10-5/6.16 × 10-5, 19.60 × 10-5/25.52 × 10-5, and 26.69 × 10-5/27.60 × 10-5 S cm-1, respectively.

When a Twist Makes a Difference: Exploring PCET and ESIPT on a Nonplanar Hydrogen-Bonded Donor-Acceptor System.

Bioinspired benzimidazole-phenol constructs with an intramolecular hydrogen bond connecting the phenol and the benzimidazole have been synthesized to study both proton-coupled electron transfer (PCET) and excited-state intramolecular proton transfer (ESIPT) processes. Strategic incorporation of a methyl group disrupts the coplanarity between the aromatic units, causing a pronounced twist, weakening the intramolecular hydrogen bond, decreasing the phenol redox potential, reducing the chemical reversibility, and quenching the fluorescence emission. Infrared spectroelectrochemistry and transient absorption spectroscopy confirm the formation of the oxidized product upon PCET and probe excited-state relaxation mechanisms, respectively. Density functional theory calculations of redox potentials corroborate the experimental findings. Additionally, time-dependent density functional theory calculations uncover the fluorescence quenching mechanism, showing that the nonradiative twisted intramolecular charge transfer state responsible for fluorescence quenching is more energetically favorable in the methyl-substituted system. Incorporating groups causing steric hindrance expands the design of biomimetic systems capable of performing both PCET and ESIPT.

Effects of functional groups on ESIPT and antioxidant activity of apigenin: A non-existent enol* state fluorescein

The development and enhancement of antioxidant drugs, which are aimed at mitigating DNA damage, mutations, and cancer, are of paramount significance in the biomedical sphere. In recent years, antioxidant drug molecules with photoluminescence have sprung up like mushrooms. Apigenin (AP), characterized by its distinctive property of excited state intramolecular proton transfer (ESIPT), plays a pivotal role in mediating antioxidant and anticancer activities. Despite being a representative molecule of the non-existent enol form (E*) state with ESIPT nature, there is a notable lack of theoretical investigations into its antioxidant properties. Herein, density functional theory (DFT) and time-dependent DFT methodologies were utilized to explore the effects of various functional groups on AP molecules in a methanol solvent. Studies have demonstrated that for the non-existent E* state fluorescence molecule AP, the ESIPT process can significantly enhance the antioxidant potency of AP and its derivatives. However, the introduction of electron-withdrawing groups significantly accelerated the ESIPT process while simultaneously suppressing the antioxidant activity of AP-CN. Conversely, the incorporation of electron-donating groups effectively inhibited the ESIPT process, yet markedly enhanced the antioxidant activity of AP-NH2. This investigation furnishes vital perspectives and sources of reference for the conception and advancement of groundbreaking antioxidant medications that aim to tackle non-existent E* state molecules.

Coordination-driven molecular switch on the base of an ESIPT-capable pyrimidine ligand: Synthesis, fine-tuning of emission by the halide anion and theoretical studies

ESIPT-based materials (ESIPT = Excited State Intramolecular Proton Transfer) find diverse applications in optoelectronics and biomedicine owing to the peculiarities of their luminescence properties. Here, an ESIPT-capable compound 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL4,2,Me) featuring a short O–H⋅⋅⋅N intramolecular hydrogen bond and two N,N-sites for metal binding has been synthesized. HL4,2,Me is the first reported molecule which can act as an ESIPT molecular switch triggered by metal ion coordination without its deprotonation. Drastic changes in the HL4,2,Me conformation in the [Zn(HL4,2,Me)X2] (X = Cl, Br,I) complexes significantly alter the photoluminescence response compared to the free ligand. In the solid state, HL4,2,Me exhibits barrierless ESIPT and large Stokes-shifted yellow-orange emission due to the interplay of anti-Kasha S2 → S0 fluorescence and Kasha-like T1 → S0 phosphorescence radiative channels. The violation of Kasha’s rule for HL4,2,Me is justified by an extraordinarily large S2 – S1 energy gap (ca. 0.9 eV), slowing down the rate of S2 → S1 internal conversion. The photoluminescence behavior of the ESIPT–incapable zinc(II) coordination compounds strongly depends on the halide anion: the chlorido complex exhibits only fluorescence, the bromido complex displays a minor phosphorescence channel in addition to a major fluorescence channel, while the iodido complex exhibits predominantly phosphorescence. As a result, the emission color of the [Zn(HL4,2,Me)X2] complexes changes gradually from blue for X = Cl to orange for X = I, providing a platform for the fine-tuning of emission by the halide anion.

Excited-State Decay and Photolysis of O-Nitrophenol after Proton Transfer. II: A Theoretical Investigation in the Microsolvated Atmospheric Environment.

Changes in atmospheric humidity affect the number of water molecules surrounding o-nitrophenol (ONP), creating an anisotropic chemical environment. It, in turn, influences the photodynamic behaviors of ONP, differing from those observed in the gas phase and in solution. Recently, we explored the excited-state decay and the generation of the hydroxyl (OH) radical before proton transfer of ONP in the microsolvated environment using the MS-CASPT2//CASSCF approach. As is well known, ONP is capable of converting to its aci-nitro isomer (aciONP) via an excited-state intramolecular proton transfer (ESIPT) process. In the present work, the photoinduced dynamics of aciONP, which can lead to an OH radical and nitrous acid (HONO), was studied using the same computational model. Our calculations demonstrated that increasing the number of water molecules affects the molecular geometries, particularly the key bond lengths and dihedral angles of the HONO group, while also reducing the relative energies of minima and intersections. Moreover, we identified two distinct types of minimum structures: one that retains the intramolecular hydrogen bond and the other that breaks the hydrogen bond with the H atom flipping outward. The latter structure, compared with the former, has a different electronic-state character and facilitates intersystem crossing processes. Subsequently, two major excited-state decay paths were proposed: (PATH I) ESIPT → S1 → S1S0 → S0; (PATH II) ESIPT → S1 → S1-2 → S1T1 → T1 → S0T1 → S0. Furthermore, the T1 state has a relatively long lifetime, allowing for the formation of the OH radical and HONO, and the corresponding energy barriers decrease as the number of water molecules increases. These theoretical findings provide valuable insights into the photodynamics of aciONP in the microsolvated atmospheric environment.

Roles of ESIPT and TICT in the Photophysical Process of a Ratiometric Fluorescence Sensor for Al3.

Precise detection of Al3+ with the aid of a fluorescence sensor is of fundamental importance in the fields of water pollution control and food safety. A comprehensive understanding of the photophysical process of the sensor as well as its underlying detection mechanism is a precondition for the design of highly efficient sensors. This contribution performs a thorough investigation of the ratiometric fluorescence sensing mechanism of an Al3+ sensor with the aid of density functional theory and time-dependent density functional theory. Two excited-state intramolecular proton transfer (ESIPT) processes are observed on the S1 state potential energy surface, which leads to emission around 565 nm. A twisted intramolecular charge transfer state is observed after one of the ESIPT processes via cis-trans isomerization of the C═N bond. However, the large energy barrier hinders its occurrence, which is quite unusual. Al3+ is found to form three strong coordination bonds with the sensor, which eliminates ESIPT processes and induces a significant blue shift of the emission spectrum to 480 nm. The origination of the sensor's selectivity is also uncovered by investigating the interaction between the sensor and interfering metal ions.

Thermo-controlled Water Microenvironment Inducing Fluorescence Enhancement of Chalcone Nanohydrogels for Mitochondrial Temperature Sensing.

Developing aggregation-induced emission (AIE)-based hydrogels that exhibit fluorescence enhancement as to thermal properties is an interesting and challenging task. In this work, we employed the fluorophore 2'-hydroxychalcone (HC), fluorescence properties of which are easily influenced by the excited-state intramolecular proton transfer and twisted intramolecular charge transfer (TICT) effects, to develop a novel type of temperature-sensitive polymers, hydroxychalcone-based polymers (HCPs). By controlling the temperature-dependent water microenvironments in HCPs, the intramolecular hydrogen bonds between water and HCPs can be regulated, thereby influencing the TICT process and leading to thermo-induced fluorescence enhancement, which shows a contrary tendency compared to typical AIEgens that always exhibit fluorescence attenuation as the thermal energy accelerates the molecular motion. Following the decoration with triphenylphosphine, the resulting polymer P-HCP assembled into nanohydrogels and served as a fluorescent probe for intracellular mitochondrial temperature sensing.

Photoexcitation Dynamics of 4-Aminopthalimide in Solution Investigated Using Femtosecond Time-Resolved Infrared Spectroscopy

Excited-state intramolecular proton transfer (ESIPT) reactions are crucial in photoresponsive materials and fluorescent markers. The fluorescent compound 4-aminophthalimide (4-AP) has been reported to exhibit solvent-assisted ESIPT in protic solvents, such as methanol, wherein the solvent interacts with 4-AP to form a six-membered hydrogen-bonded ring that is strengthened upon excitation. Although the controversial observation of ESIPT in 4-AP has been extensively studied, the molecular mechanism has yet to be fully explored. In this study, femtosecond infrared spectroscopy was used to investigate the dynamics of 4-AP in methanol and acetonitrile after excitation at 350 and 300 nm, which promoted 4-AP to the S1 and S2 states, respectively. The excited 4-AP in the S1 state relaxed to the ground state, while 4-AP in the S2 state relaxed via the S1 state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S1 state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S1 state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S1 state. Upon the excitation of 4-AP to the S2 state, the transition to Enol-4-AP in the S1 state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S2 to the S1 state. The internal S2 → S1 conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S2 state. The detailed excitation dynamics of 4-AP clearly reveal the molecular mechanism underlying its negligible ESIPT, despite the fact that it forms a favorable structure for solvent-assisted ESIPT.

One molecule to couple them all: Toward realistic numbers of molecules in multiscale molecular dynamics simulations of exciton-polaritons.

Collective strong coupling of many molecules to the confined light modes of an optical resonator can influence the photochemistry of these molecules, but the origin of this effect is not yet fully understood. To provide atomistic insights, several approaches have been developed based on quantum chemistry or molecular dynamics methods. However, most of these methods rely on coupling a few molecules (or sometimes only one) to a single cavity mode. To reach the strong coupling regime with such a small number of molecules, much larger vacuum field strengths are employed than in experiments. To keep the vacuum field realistic and avoid potential artefacts, the number of coupled molecules should be significantly increased instead, but that is not always possible due to restrictions on computational hardware and software. To overcome this barrier and model the dynamics of an arbitrarily large ensemble of molecules coupled to realistic cavity fields in atomistic molecular dynamics simulations, we propose to coarse-grain subsets of molecules into one or more effective supermolecules with an enhanced dipole moment and concerted dynamics. To verify the validity of the proposed multiscale model, we performed simulations in which we investigated how the number of molecules that are coupled to the cavity affects excited-state intra-molecular proton transfer, polariton relaxation, and exciton transport.

ESIPT-induced intersystem crossing leads to tautomer fluorescence quenching for 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one molecule

The excited-state intramolecular proton-transfer (ESIPT) dynamics of 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF) and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) have been investigated using time-dependent density functional theory (TDDFT). Upon photoexcitation, 3NTF exhibits a single fluorescence emission while 3FTF is fluorescence quenched when dissolved in cyclohexane solution. The present study reveals that both species undergo barrierless ESIPT process, and the underlying reason for fluorescence quenching in 3FTF has been elucidated. Specifically, it is concluded that intersystem crossing (ISC) is responsible for the fluorescence quenching in the 3FTF molecule due to the energy gap between the S1 and T2 states is only 0.12 eV plus large S1 → T2 spin–orbit coupling resulting in a strong interaction between the singlet and triplet states. The present study provides a reference for the fluorescence quenching associated with thiol-hydrogen bond molecules, and it is helpful for further research on ESIPT reactions of sulfur-containing molecules.

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Excited‐state intramolecular proton transfer (ESIPT) enables the rearrangement of photoinduced charge distribution and results in the structural isomerization in the excited state. The EISPT mechanism can be used for optimizing the photocatalytic performances of β‐ketoenamine‐linked two‐dimensional covalent organic frameworks (COFs). It has been demonstrated that the modification of building blocks with electron‐withdrawing cyano‐groups benefits the ESIPT‐induced photoisomerization for COFs, thereby promoting photocatalytic activity and achieving the excellent hydrogen evolution performances from water splitting. More details are discussed in the article by Guo et al. on pages 2621—2626.image

Excited-state intramolecular proton transfer (ESIPT) salicylaldehyde Schiff bases: ratiometric sensing of ammonia and biologically relevant ions in solution and solid state

ABSTRACT The intricate molecular architecture of ESIPT salicylaldehyde Schiff bases facilitates dynamic processes, inducing tunable photoluminescent properties. Notably, their halochromic nature, exhibiting colour changes in response to external stimuli, adds a vibrant dimension to their molecular repertoire. This sensitivity extends to environmental factors, making them valuable indicators for alterations in surroundings. The compound (E)-N’-(3,5-dibromo-2-hydroxybenzylidene)-4-methylbenzohydrazide (PTBH) demonstrates exceptional sensitivity to ammonia, enabling real-time detection (LOD = 0.14 nM) in both solution (ratiometric) and the solid state. Moreover, their metal chelation capability allows simultaneous sensing of Mg2+ and Fe2+ ions, addressing environmental hazards. Exploiting molecular recognition, the fluorescent probe serves as sensors for amino acids, opening new avenues in biomedical diagnostics. The study introduces a novel solid-state emissive Schiff base, highlighting its stimuli-responsive photoluminescent properties and diverse applications, emphasising its potential in intelligent fluorescent materials for analytical and sensing technologies.

Flexible organic crystals with multi-stimuli-responsive CPL for broadband multicolor optical waveguides.

Flexible organic crystals, capable of transmitting light and responding to various external stimuli, are emerging as a new frontier in optoelectronic materials. They hold immense potential for applications in molecular machines, sensors, displays, and intelligent devices. Here, we report on flexible organic crystals based on single-component enantiomeric organic compounds, demonstrating multi-stimuli-responsive circularly polarized light (CPL). These crystals exhibit remarkable elasticity, responsiveness to light and acid vapors, and tunable circularly polarized optical signals. Upon exposure to acid vapors, the fluorescence of the crystals shifts from initial yellow emission to green emission, attributable to the protonation-induced inhibition of excited-state intramolecular proton transfer. Under UV irradiation, the fluorescence emission undergoes a red-shift, resulting from the molecular transformation from an enol configuration to a ketone configuration. Notably, both processes are reversible and can be restored under daylight. The integration of reversible fluorescence changes under light and acid vapors stimuli, CPL signals, and flexible optical waveguides within a single crystal paves the way for the application of organic crystals as all-organic chiral functional materials.

Theoretical Investigation on Proton Transfer Directionality and Dynamics Behavior of 3-(Benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde with Two Asymmetric Proton Acceptors.

A detailed theoretical investigation on the excited state intramolecular proton transfer (ESIPT) directionality and dynamics behavior of 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with two unsymmetric proton acceptors (N and O2) has been performed. The hydrogen bond O1-H···N in BTHMB-a formed by the O1-H group with the N atom or O1-H···O2 in BTHMB-b formed by the O1-H group with the O2 atom is enhanced upon photoexcitation, and the strength of the O1-H···N bond is stronger, which will drive the O1-H proton to the N atom. Potential energy curves further confirm that ESIPT occurs in the N atom because of the smaller energy barrier (0.39 kcal/mol). Results of dynamics simulations manifest that no surface hopping exists between the S0 and S1 states within 300 fs, and ESIPT time constants of BTHMB-a and BTHMB-b are 48 and 151 fs, respectively. While the reverse ESIPT is observed in BTHMB-b at 294 fs, implying that the O1-H proton is transferred to the N atom instead of the O2 atom. The consistency of the calculated absorption (390 nm) and fluorescence spectra (443 and 602 nm) of BTHMB-a with the experimental values (390, 410, and 605 nm) confirms this conclusion again. The charge distribution analysis shows that the charge on the proton acceptors increases, and the O2 atom has higher electronegativity because it has more negative charges. The minimum surface electrostatic potential on the N atom in BTHMB-b correlating with the pKb value is -47.38 kcal/mol, indicating that the N atom has strong basicity. Therefore, the basicity of the N atom dominates the ESIPT process rather than the electronegativity of the O2 atom.

A dual-response fluorescent hemicyanine probe for the detection of mitochondrial hypochlorite and viscosity based on ESIPT/AIE and TICT

Viscosity and hypochlorite (ClO−) are two important factors affecting the physiological activity and function of mitochondria. Hypochlorite (ClO−) is a vital reactive oxygen species, and endogenous ClO− acts as an important germicidal oxidant in the immune system at low concentrations. While, fluctuations in viscosity can cause mitochondrial dysfunction that is linked to several diseases. We herein report on a hemicyanine-based probe (BDTB-PA) for the detection of hypochlorite (ClO−) and viscosity. The probe exhibits a very low fluorescence background in an aqueous solution due to twisted intramolecular charge transfer (TICT) and blocked excited-state intramolecular proton transfer (ESIPT). In a viscous solution, the TICT of the probe was inhibited, generating a strong emission at 580 nm. In contrast, upon treatment with ClO−, the phenylboronic acid pinacol ester group of probe BDTB-PA was removed, and the hydroxyl group was released, activating the ESIPT process. Coupled with its aggregation-induced emission (AIE) characteristics, the product of BDTB-PA with ClO− exhibited a distinct emission at 540 nm. As such a solution of the probe BDTB-PA exhibited a distinct fluorescent color change from colourless to green under UV light with the addition of ClO−. More importantly, probe BDTB-PA was used for monitoring ClO− and viscosity in the mitochondria of living cells, plants and zebrafish.

Visible‐Light‐Driven Solid‐State Fluorescent Photoswitches for High‐Level Information Encryption

Developing visible‐light‐driven fluorescent photoswitches in solid state remains an enormous challenge in smart materials. Such photoswitches are obtained from salicylaldimines through excited‐state intramolecular proton transfer (ESIPT) and subsequent cis‐trans isomerization strategies. By incorporating a bulky naphthalimide fluorophore into Schiff base, three photoswitches achieve dual‐mode changes (both color and fluorescence) in the solid state. In particular, the optimal one generates triple fluorescence changing from green, to yellow and finally orange upon visible light irradiation. This switching process is fully reversible and can be repeated at least 10 times without obvious attenuation, suggesting its superior photo‐fatigue resistance. Mechanism studies reveal that naphthalimide group not only enables the tuning of multicolor with an additional emission, but also induces a folded structure, reducing molecular stacking and facilitating ESIPT and cis‐trans isomerization. As such, photopatterning, ternary encoding and transient information recording and erasing are successfully developed. The present study provides a reliable strategy for visible‐light‐driven fluorescent photoswitches, showing implications for advanced information encryption materials.

A ‘Turn-on’ fluorogenic probe for selective and specific detection of Hg(II) ions

Mercury (Hg2+) and its associated compounds have drawn serious concern from the scientific communities for their extreme toxicity to human beings. The present article introduced a chromone-benzoxazole embracing an effective fluorogenic probe, (E)-3-(((4-(benzo[d]oxazol-2-yl) phenyl) imino) methyl)-2-methoxy-2H-chromen-4-ol (BPMC) for selective and specific detection of Hg2+ ions having detection and quantification limit in the µM range. A significant photoluminescence enhancement has been observed from BPMC solution (water-DMSO mixture, 50 % v/v) displaying low to highly intense blue-violet photoluminescence under a 365 nm UV lamp due to the inhibition of intramolecular charge transfer (ICT) and excited state intramolecular proton transfer (ESIPT) processes involved in BPMC. Furthermore, to achieve on-site detection of Hg2+ ions and investigate the practical utility of our developed probe, we have fabricated a BPMC-stained paper-strips-based test kit and demonstrated its practical effectiveness for on-spot detection. The details of the detection mechanism have been revealed through 1H NMR, FT-IR, and high-resolution mass spectrometric analysis. The present report evokes a broader perspective for tailoring ICT and ESIPT-based chromo-fluorogenic probes to accelerate their real-world application and environmental monitoring.

Color-tunable and white circularly polarized luminescence through confining guests into chiral MOFs.

Herein, chiral metal-organic frameworks (MOFs), DCF-20 and LCF-20, were utilized as matrices for both chirality transfer and energy transfer. HBT1@MOFs and HBT2@MOFs emit excitation-dependent circularly polarized luminescence (CPL) due to excited-state intramolecular proton transfer (ESIPT). HBT1/C152/NIR@MOFs exhibit full-color and white CPL. The luminescence dissymmetry factors (glum) were significantly increased, benefiting from the efficient chirality space transfer and high luminescence efficiency.

Theoretical study on precise regulation of ESIPT reaction based on ICT effect of o-carborane derivatives

The unique stereocarborane fluorophore design has attracted widespread attention because of its ability to effectively enhance the performance of thin-film fluorescence sensors. This study provides a detailed theoretical investigation of the excited-state intramolecular proton transfer (ESIPT) reactions of three o-carborane derivatives, NaCBO, PaCBO and PyCBO, using density functional theory and time-dependent density functional theory. We found that as the number of benzene rings in the electron donor groups increases, the interaction between the first excited-state electron donor–acceptor fragments is enhanced, weakening the intramolecular hydrogen bonding strength, which in turn inhibits the ESIPT. Notice that the intramolecular charge transfer (ICT) is proportional to the interaction between electron donor and acceptor fragments with the analysis of electron structure. The higher the ICT level of the target system, the more unstable the keto form generated by ESIPT. Therefore, the ESIPT can be precisely manipulated by rationally adjusting the ICT effect. Furthermore, with increasing solvent polarity, the photophysical properties of the chromophores exhibit non-solvatochromism. This is attributed to the rigid o-carborane structure, which serves as a spatial scaffold to increase the adaptability of the molecule to the environment. This study is expected to provide theoretical guidance and new ideas for preparing high-performance thin-film fluorescence sensing.

Constructing ESIPT-Capable α-Cyanostilbene Luminogens: Influence of Different Aromatic Substitutions Tethered to H-Acceptor (CH = N) on Photophysical Properties, Cu2+ and Fe3+ Detection.

Herein, two excited-state intramolecular proton transfer (ESIPT)-capable α-cyanostilbene luminogens were synthesized by Schiff base reaction of salicylaldehyde-like α-cyanostilbene candidate with 1-naphthylamine and 3-biphenylamine, respectively. We systematically analyzed their photophysical properties compared with their analogue, and demonstrated that their fluorescence behaviors could be elaborately modulated by different aromatic substitutions tethered to H-acceptor (CH = N). In virtue of the outstanding solid fluorescence, the 3-biphenylamine-decorated fluorophore was applied for monitoring Cu2+/Fe3+ qualitatively on the TLC-based test strip in real time and sensing Cu2+/Fe3+ quantitatively in the THF/H2O medium (fw = 90%, pH = 7.4). When the probe chelated with Cu2+/Fe3+, similar "turn-off" fluorescence signal outputs were triggered. From the fluorescence titration experiments, the detection limits were evaluated as 7.97 × 10- 8 M for Cu2+ and 8.24 × 10- 8 M for Fe3+, and the binding constant (Kα) values of the complexes were found to be 7.80 × 104 M-1 for Cu2+ and 9.06 × 104 M-1 for Fe3+. Job's plots indicated that probe complexed with Cu2+/Fe3+ in a 2:1 binding stoichiometry ratio. Furthermore, the probe was used to accurately quantify the Fe3+ spiked in real water specimens. This study offered a new perspective to construct ESIPT-capable α-cyanostilbene luminogen as the potential luminescent probe.